1. Field of the Invention
The present invention relates to an optoelectric composite substrate and a method of manufacturing the same and, more particularly, an optoelectric composite substrate constructed to have electric signal wirings and light signal waveguides and a method of manufacturing the same.
2. Description of the Related Art
Recently it is the electric wirings in the information terminal that are forming a bottleneck while the provision of trunk communication lines is making steadily progress mainly based on the optical fiber communication technology. In order to pass the limit of the transfer rate of the electric signal, the optoelectric composite substrate of the type that the signal transmission in the high-speed area is executed by the light signal has been proposed against such background, in place of the conventional electric circuit substrate in which all signal transmissions are executed by the electric signal.
As shown in FIG. 1, in the optoelectric composite substrate in the prior art, an optical waveguide 106 having such a configuration that a core portion 102 is surrounded by a cladding portion 104 is pasted onto a lower circuit board 100 by an adhesive layer 108. Also, an upper circuit board 200 is pasted onto the optical waveguide 106 by a transparent adhesive layer 108a. Connection pads 202 to which the optical device is connected electrically are provided to predetermined portions of the upper circuit board 200. The optical waveguide 106 is provided to extend in the direction perpendicular to a sheet of the drawing of FIG. 1.
Also, through holes 300 are provided in portions of the upper circuit board 200, the cladding portion 104, and the lower circuit board 100 under the connection pads 202 respectively.
A through hole conductive film 302 is formed on inner surfaces of the through holes 300, and a resin body 304 is filled in the through holes 300. The through hole conductive film 302 is connected to the connection pad 202 on the upper circuit board 200 and also is connected to a wiring layer (not shown) built in the lower circuit board 100.
Also, an opening portion 200a is provided in a portion of the upper circuit board 200 over an area that contains the core portion 102 of the optical waveguide 106. In addition, terminals of an optical device 400 are connected electrically to the connection pads 202 of the upper circuit board 200. The optical device 400 (e.g., light emitting element) is driven by the electric signal that is supplied from the lower circuit board 100 via the through hole conductive film 302, and then a light emitted from its light emitting surface (lower surface) is incident on a light incident portion A of the optical waveguide 106 via the opening portion 200a of the upper circuit board 200. Then, light signal incident upon the core portion 102 is propagated by repeating a total reflection, then input into a photodetector arranged on the other end of the optical waveguide 106, and then converted to the electric signal once again.
The optoelectric composite substrate similar to such configuration is set forth in Patent Literature 1 (Patent Application Publication (KOKAI) 2003-287637), for example.
In the meanwhile, in order to get the good optical coupling characteristic between the optical device 400 and the optical waveguide 106, first it is requested that a discrepancy between the light emitting portion of the optical device 400 and the light incident portion A of the optical waveguide 106 in the x-y directions (the horizontal direction in FIG. 1) is made small. In other words, it is extremely important that the light emitted from the optical device 400 is incident on the light incident portion A of the optical waveguide 106 in alignment with its inner side without leakage to the outer side.
Also, second a distance between the optical device 400 and the optical waveguide 106 (the z direction in FIG. 1) constitutes an important factor. In other words, since a width of a luminous flux incident upon the light incident portion A is expanded wider as a distance between the light emitting surface (lower surface) of the optical device 400 and the optical waveguide 106 is extended longer, an area of the light incident portion A must be set larger with the increase of the distance than it is needed. Since there is a limit to the area of the light incident portion A of the optical waveguide 106, it is extremely important that the distance between the optical device 400 and the optical waveguide 106 is set short.
For example, as shown in FIG. 2, in case a radiation angle θ of the light from the optical device 400 is set to 23 and an area of the light incident portion A of the optical waveguide 106 is set to 35×35 μm2, a distance d between the optical device 400 and the optical waveguide 106 is given as 87.5 μm when a width w of the luminous flux of the optical device 400 is equal to the light incident portion A of the optical waveguide 106. In this case, the case where no positional discrepancy between the light emitting portion of the optical device 400 and the light incident portion A of the optical waveguide 106 in the x-y directions is present is illustrated in FIG. 2.
In addition, as shown in FIG. 3, if the distance d between the optical device 400 and the optical waveguide 106 is set shorter than that in FIG. 2, the width w of the luminous flux incident upon the light incident portion A is narrowed and thus the light is incident on the inner side of the light incident portion A. Therefore, a tolerance of the displacement in the x-y directions can be set large.
As described above, it is understood that, if the distance d between the optical device 400 and the optical waveguide 106 is set short, not only the area of the light incident portion A of the optical waveguide 106 can be reduced but also the tolerance of the displacement in the x-y directions can be set large.
In the above prior art, since the upper circuit board 200 having the connection pads 202 thereon and the optical waveguide 106 are pasted together via the adhesive layer 108a, a precise alignment between the connection pads 202 on which the optical device 400 is mounted and the light incident portion A of the optical waveguide 106 is difficult and thus it is difficult to set a mutual positional relationship. As a result, it is supposed that the positional discrepancy between the light from the optical device 400 and the light incident portion A of the optical waveguide 106 in the x-y directions is ready to occur and thus the desired optical coupling characteristics cannot be obtained.
Further, since the optical device 400 is mounted on the upper circuit board 200, the optical device 400 is away from the optical waveguide 106 by thicknesses of the upper circuit board 200 having the connection pads 202 thereon and the adhesive layer 108a. Therefore, when the area of the light incident portion A of the optical waveguide 106 is reduced, the luminous flux from the light emitting element 400 is incident on the outside of the light incident portion A. As a result, there exists such a problem that the desired optical coupling characteristics cannot be obtained.
In above Patent Literature 1, no consideration is given to these problems.